1. Field of the Invention
This invention pertains to semiconductor lasers and, more specifically, to semiconductor lasers having a broad spectrum of emission.
2. Description of Related Art
Although narrow-band spectral output is often appreciated for a laser, a lasing spectrum which has a broad spectral bandwidth is desirable for certain applications. In particular, a laser spectrum with many longitudinal modes is quite desirable for a mode-locked laser because the pulse width is, in general, inverse to the spectral bandwidth. Also, a broadband laser spectrum is useful in optical communication systems provided that the output radiation is spectrally split into several independent channels. Such a system can be based on a single broadband laser and, therefore, can be advantageous in terms of fabrication simplicity and lower cost compared to more common wavelength division multiplexing (WDM) systems based on several lasers, in which each wavelength channel requires its own laser source.
Optical generating devices with a very broad spectrum of optical emission, the so-called super-continuum, are known in the art. These devices usually exploit the propagation of a short optical pulse through a sufficiently long strongly nonlinear optical substance, e.g. optical fiber, as described, for example, in U.S. Pat. No. 6,813,423. Although the spectral range of emission can be very large (e.g. several hundred nanometers), such devices are usually not very compact.
A light-emitting diode (LED) and a superluminescent light-emitting diode (SLED) are capable of emitting in a broad spectral range. However, these devices are typically characterized by low efficiency compared to a laser device, and their output power is typically low. Therefore, when used as an optical source for a WDM system, a LED or a SLED only provides limited power per spectral channel of the WDM system.
An example of a broadband laser source is disclosed in U.S. Pat. No. 6,628,686. This patent describes a laser that has an InGaAsP active structure with modified effective bandgap energy. The spatially varying emission spectrum allows emission at multiple wavelengths or emission in a broad band. This solution, however, suffers from complexity in the fabrication method, which exploits post-growth modification of bandgap properties by rapid thermal annealing, as disclosed in U.S. Pat. No. 6,611,007.
Because the bandwidth of the emission spectrum of the laser is fundamentally limited by the width of the optical gain spectrum, it is desirable for a broadband laser to include an active region with a broad gain spectrum. This is easily achieved with a quantum dot array. A quantum dot is a three-dimensional semiconductor structure which has a size of the order of a de-Broglie wavelength, thereby producing quantization of the energy levels of the confined electrons and holes. Stranski-Krastanow quantum dots, also known as self-organized quantum dots (hereinafter quantum dots), have appeared recently as a practical realization of ideal quantum dots.
Quantum dots formed by self-organization epitaxial methods are typically characterized by the inhomogeneous broadening of quantum states caused by inevitable non-uniformities of dimensions, chemical composition, shape, strain, as well as other parameters of quantum dots affecting the quantum state energies in quantum dots. Although a high degree of uniformity is usually appreciated in quantum dots which are intended for use in light-emitting devices, a certain amount of non-uniformity can provide a significant inhomogeneous broadening of quantum dot states and, therefore, a broad gain spectrum under certain pump levels.
Other properties of quantum dots when used in a laser's active region including, but not limited to, low threshold current density, low alpha-factor, reduced temperature sensitivity, and extended wavelength range of emission, also make a quantum dot laser advantageous over a quantum well laser for certain applications.
U.S. Pat. No. 6,768,754 discloses a tunable laser system, which includes a quantum dot laser active region with a gain spectrum that extends continuously over a broad wavelength range of at least one hundred nanometers. Another example of a tunable laser system, U.S. Pat. No. 6,816,525, describes a method that is capable of producing inhomogeneous broadening of the optical gain spectrum, which is beneficial for tunable lasers and arrays of lasers that have a wide range of operating wavelength. A tunable laser of the prior art typically included a quantum dot active region, which is capable of providing a sufficient optical gain in a wide optical band, and a wavelength-selective element (e.g., an element having a reflectivity that is a function of wavelength) for selecting a wavelength of interest emitted by the laser diode.
One benefit of prior art quantum dot tunable lasers is that the large tuning range of the quantum dot active region permits a multi-wavelength laser array to be fabricated from a single quantum dot laser wafer, the array having a large number of different operation wavelengths for wavelength division multiplexed applications. One disadvantage of prior art multi-wavelength laser arrays is that an optical source for an N-channel WDM system should include at least N laser devices. This results in additional complexity in fabrication and additional expense. Therefore, there is a need in the art for a compact and inexpensive optical source for a WDM system which includes only a single laser device.